The problem is as you said “The box wouldn’t look like a frustum”. Maybe we are taking about different things here. I mean if you drag a box on top of the HUD, as in this picture link. With a box created from the start point to the end point in world coordinates, the box would potentially cover more “units” than the box displayed on the HUD. This is since my camera isn’t orthogonal. The greater the FOV is, the greater error. The 3D representation of the HUD box would indeed look like some frustum shape, unless we use an orthogonal perspective.
